The present invention relates to mounting structures for integrated circuit chips and more particularly to packages for mounting multichip modules on printed circuit boards.
It has become increasingly common to use multichip modules to mount subassemblies of integrated circuit chips on printed circuit boards. Such modules frequently include several integrated circuit chips supported on a planar silicon base which includes a fine-line multilayer structure for electrically interconnecting the chips. Most multichip modules are enclosed within packages which are intended to protect them from adverse environmental influences such as moisture and allow for the modules to be mechanically attached and electrically connected to circuit boards. In the past, the packages for multichip modules have generally been metal casings of the type used in thick and thin film hybrid microelectronics having small apertures within their side walls for allowing glass encapsulated leads to pass through the walls of the casings. The leads have served both to mechanically attach the package to the circuit board and electrically connect the module to the circuit board. The performance of such packages has been poor on account of the delicate nature of this method of mechanical attachment and mismatches between the coefficients of thermal expansion of the components of the package and the circuit board on which the package is mounted. In particular, the leads extending out from the package have been a design problem because, if they are chosen to be thin and compliant, they provide little protection against vibration and shock while, if they are chosen to be thick and stiff, the solder joints by which they are connected to the package and circuit board are subject to thermal cycling problems resulting in cracking and breakage. For these and other reasons, it has proven very difficult to design effective and durable packages for multichip modules.
It is therefore an object of the present invention to provide a package for a multichip module which is rugged and durable and is able to withstand shock and vibration without overstressing while also having a long thermal cycle life.
It is another object of the present invention to provide a package for a multichip module which provides accurate positioning of electrical leads with good lead-to-pad alignment as well as electrical lead co-planarity for reliable solder joint formation.
It is a further object of the present invention to provide a package for a multichip module in which the components of the package are matched with respect to their thermal expansion characteristics and in which appropriate interfaces are provided between components which are not matched with respect to their thermal expansion characteristics.
It is yet another object of the present invention to provide a package for a multichip module which has good hermeticity and hermeticity retention to protect against environmental influences, good thermal conductivity to keep IC junction temperatures low and good accessibility to the multichip module to allow for convenient repair of the module.
It is yet a further object of the present invention to provide a package for multichip modules which has the above-referenced desirable characteristics yet is economic to manufacture both for applications where full hermeticity is required and applications where only limited protection is required against adverse environmental conditions.